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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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190                               ALASKAN GLACIER STUDIES
transformation of the advancing Yakutat Bay glaciers it might be urged against the theory of advance under the impulse of earthquake shaking that the fact that analogous advance in other regions had not been observed was opposed to this theory. Such an objection •would have some force if the advance were the result of ordinary changes, to be commonly expected in regions of glaciation; but it lacks force in view of the exceptional conditions under which such an advance is possible. Only by the combination of vigorous earth shaking and heavy snow cover in unstable positions, can such a notable advance be brought about. The latter condition is commonly present in the higher mountains from which good-sized glaciers flow, though rarely in such a degree as in the Alaskan coast ranges; but vigorous earth shaking is not so common. Even in Alaska, a region of abundant earthquakes, such shaking as that of 1899 must be rare; and although we have record of many earthquakes, both before and after 1899, we have heard of no shaking in Alaska between 1788 and 1918 that equals this period either in intensity or in duration. The Alps, whose glaciers have beeri studied with greater care and for a longer time than any other region, is not a section of abundant, great earthquakes. In other regions of earthquake-shaken mountains, like the Caucasus and the Himalayas, the glaciers have been little studied. We hold, therefore, that in some glacial regions an advance even approximating that of the Yakutat Bay glaciers is not to be expected; and even in those regions where a similar transformation may be looked for, it could come only at intervals many years apart. It is our opinion, that the phenomenon, in anything like the degree observed in the Yakutat Bay region, is of rare occurrence, which is the main reason why it has not hitherto been observed.
There is the further fact that the cycle of the advance is completed in such a brief period of time that an observer must be on hand at just the right time to detect it, and he must have knowledge of the previous condition of the glacier and the position of its front. For instance, if we had not had Russell's description and photographs of 1890 and 1891 we would not have known of the change in Galiano Glacier; had Gilbert not observed Variegated Glacier in 1899 and we in 1905, we would in 1906 have naturally assumed that the broken condition was normal to it. Similar statements might be made about the other advancing glaciers. Moreover, even if we had previous observations as a basis for comparison, since the advance stops so abruptly, in a year or less, not going on for several years, as under normal climatic control, and since ablation so quickly heals the broken surface, a cycle of advance might "pass completely without detection, if the period between observations were five or ten years. It is also to be noted, that if a glacier surface were normally cre-vassed, as Hubbard, Turner, and Nunatak Glaciers are, the evidence of change would not be so easy of detection as in the uncrevassed, stagnant glacier ends.
By good fortune our expeditions to Yakutat Bay were timed just right for the detection of the advances, and they had the advantage of having available the results of the previous expeditions by Russell and Gilbert. There may have been similar advances in other remote regions of heavy snowfall and great earthquakes, say in Alaska, or the Caucasus, or the Himalayas, which have failed of detection because of the lack of successive expeditions properly timed to note the evidences of change. Hence, not only are there few regions where spasmodic advance of glaciers in response to earthquake shaking is to be expected, and even there only at long-separated intervals; but the chance of discovery of the evidence of such advance, and the correlation of it with